Copolymers of vinyl chloride and maleic acid esters and/or fumaric acid esters are known. They are characterized, even in the presence of plasticizers, by good workability, e.g., fluidity in the melted state, so that moldings, sheets, wire insulation or the like can be made better than they can from polyvinyl chloride homopolymers. See, for example, Kainer, "Polyvinylchlorid und vinylchlorid-Mischpolymerisate," 1965, p. 105, and British Pat. Nos. 466,898 and 487,593. The products made from the copolymers, however, are inferior to those made from polyvinyl chloride homopolymers, for example in regard to their thermal stability. There is therefore a need to compensate for this disadvantage.